In U.S. Pat. No. 4,255,666 owned by the present assignee, a two-stage, proximity type image intensifier is described. This device incorporated two stages of amplification in an effort to provide improved gain over that of a single-stage device described in U.S. Pat. No. 4,140,900 also owned by the present assignee. Both U.S. Pat. Nos. 4,140,900 and 4,255,666 are hereby expressly incorporated herein by reference.
The two stage device described in U.S. Pat. No. 4,255,666 incorporates a flat scintillator screen, an output display screen and an amplification means intermediate to the scintillator screen and the output display screen. The two stage image intensifier tube comprises a metallic vacuum tube envelope and a metallic, inwardly concave input window.
In operation, an x-ray source generates a beam of x-rays which passes through a patient's body and casts a shadow onto the input window of the tube. The x-ray image passes through the input window and impinges upon the flat scintillation screen which is deposited on an aluminum substrate. The scintillation screen converts the x-ray image into a light image. This light image is "contact transferred" directly to an immediately adjacent first photocathode layer which converts the light image into a pattern of electrons. The scintillation screen and photocathode layer comprise a complete assembly.
A first phosphor display screen is mounted on one face of a fiber optic plate which is suspended from the tube envelope by means of insulators. On the opposite face of the fiber optic plate a second photocathode is deposited. The fiber optic plate is oriented in a plane substantially parallel to the plane of the scintillation screen.
A second phosphor display screen is deposited on an output window. A high voltage power supply is connected between the first phosphor display screen and the first photocathode as well as between the second photocathode and the second phosphor display screen. The power supply provides approximately 15 kV to each stage (approximately 30 kV total). The first display screen and the second photocathode are connected together and operate at the same potential.
In operation, the electron pattern on the negatively charged first photocathode layer is accelerated towards the first, positively charged (relative to the photocathode layer) phosphor display screen by means of the electrostatic potential supplied by the high voltage source connected between the display screen and the photocathode screen. The electrons striking the display screen produce a corresponding light image which passes through the fiber optic plate to impinge on the second photocathode. The second photocathode then emits a corresponding pattern of electrons which are accelerated toward the second phosphor display screen to produce an output light image which is viewable through the output window.
While the tow-stage device described above did achieve fundamental performance improvements in gain as well as other parameters over the single-stage device, it still did not achieve the performance of conventional inverter type x-ray image intensifiers. Performance of the two-stage device is found to fall short in three distinct areas: brightness gain, contrast ratio and limiting resolution.
The two-stage device has a conversion brightness of approximately one-third that of conventional inverter type tubes. This difference is due in part to the fact that the two-stage device is a unity magnification device while conventional inverter type tubes are typically .times.10 demagnification devices. This difference translates directly to a 100 fold increase in conversion gain. The image size of the inverter type tube is however only 1/10th that of the two-stage device.
The two-stage device did achieve a threefold increase in gain over the single-stage device by the incorporation of the fiber optic element. This element, however, added significantly to the cost of the device, increased its overall weight and reduced its ruggedness as well. Further increases in gain have not been achieved due to the prohibitive cost of providing additional stages of amplification or the inability to further optimize the efficiency of the various layers which comprise the two-stage device.
Image contrast of the two-stage device has also been found inferior to the conventional inverter type tubes. Typically large area contrast ratios for the inverter tubes are better than 20:1 while the two-stage device exhibits a 15:1 contrast ratio. The loss of image contrast in the two-stage device is primarily due to reflected light and backscattered electrons within the space between the photocathode and phosphor layers. In inverter type tubes the same problems exist but to a lesser degree since the large space between the single photocathode and phosphor layers allow for a substantial amount of dispersion. Attempts to improve the performance of the two-stage device through the incorporation of antireflection layers and optimization of the aluminum layer coatings on the phosphor screens have rarely achieved the 20:1 contrast of the inverter type tubes.
Resolution is a measure of how faithfully an optical device reproduces detail. In this respect, the two-stage device suffers in performance by up to 30% due largely to the extreme sensitivity of its proximity focussing technique to the surface texture of the cesium iodide scintillator. This degradation is compounded by optical and x-ray scattering within the scintillator. Thinner scintillators or scintillators composed of finer crystals could offer improvements. However, thinner crystals reduce scintillator efficiency and gain while a finer crystal structure further roughens the surface.
It is therefore an object of this invention to overcome the above referenced problems and others by providing an improved panel type image intensifier tube whose performance is comparable to that of conventional inverter type tubes.